If border posts return to Northern Ireland they will be attacked, according to the Taoiseach.
Leo Varadkar says they'll become natural targets for people if they have to be re-established post-Brexit.
But he believes there won't be a return to the violence of the past in Northern Ireland.
Mr Varadkar says certain tensions could be re-ignited by a hard border.
"I don't anticipate a return to violence like the kind we had 20 or 30 years ago.
"But if you do have physical infrastructure - if you do have cameras, and signs and border posts - those things will become targets.
"And then what do you do? Do you accept that, or do you then put in a guard to stop them doing that.
"And that's why it might all escalate and go the wrong way".